Genetic light switches show that fMRI = brain activity

By switching neurons on and off with beams of light, brain scientists have attempted to settle a question that has long dogged their field: are functional-MRI brain scans, which measure blood flow, really a snapshot of firing neurons?

The answer seems to be "yes". "It places the fMRI field on a sound footing," says Karl Deisseroth, a neurologist at Stanford University in California who led the latest study. However, others say fMRI signals can occur when neurons do not fire.

Since the technique was first developed in the early 1990s, research with fMRI scanners has spawned tens of thousands of papers that rely on the assumption that signals captured in the scanners are the result of activity in so-called excitatory neurons, which underlie communication between brain cells, both near and far.

But as fMRI measures only the flow of oxygenated blood, not electrical or molecular changes in the brain, some have suggested that other processes aside from, or as well as, neural excitation might cause fMRI signals.

Light switching

In an attempt to settle the question, Deisseroth's team turned to a relatively new technique called optogenetics, in which a handful of neurons can be switched on and off with light. It is the perfect tool to test whether stronger fMRI signals correspond to increases in neural activation, says Deisseroth.

His team inserted a gene into the brains of sedated rats that makes some of their neurons fire in the presence of light. When the researchers activated these neurons in a brain area that controls movement, an fMRI machine detected a signal in that same area.

What's more, the signal's activity patterns were similar to those that neuroscientists observe in humans as they perform various activities in brain scanners, Deissoroth says. This suggests that the activity of these neurons underlies fMRI.

The case isn't quite closed, however. The results don't prove that such neural activity is the only thing that can cause neurons to "light up" in an fMRI scan, points out Aniruddha Das at Columbia University in New York.

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